Under the big, bright sky

When sleeping under the stars, important questions can be asked. Try this one on for size.

Q. How big is the universe? One could also philosophically ask, How small are we?

A. Earth seems like a tiny speck of dust compared to the vastness of the Milky Way Galaxy. Earth, with its 6 billion human passengers, spins as it circles the sun with the rest of the local planets. The solar system in turn hurtles around in a lopsided loop around the galaxy, moving at about 150 miles a second. Despite that rate, the sun and the planets take about 250 million years to complete one circuit of the galactic hub.

How can we tell how vast the universe is? There are several methods. Lacking a tape measure long enough, we have a very accurate method known as parallax, which detects how a foreground star or something in the solar system has shifted in respect to much farther background stars. To do this, we need to know the length of the base line where the different observations are made.

If you have two observers only hundreds of miles apart and photograph the moon at the same instant, one can readily tell the shift of the moon’s placement next to any stars in the background. Employing trigonometry, we can then measure the distance to the moon.

To find the distance of a star, astronomers use the diameter of Earth’s orbit as a base line — from where Earth is one date to the opposite point on the other side of the sun, a half year later. This is approximately 186 million miles. Measuring the varying angles of observation to the star in question, we can detect the parallax shift for stars as far as about 400 light years.

That’s how far light travels in 400 years, which is about 2,346,000,000,000,000 miles. Four hundred light years, however, is still in the sun’s neighborhood. The entire Milky Way is about 120,000 light years across and about 12,000 light years thick at the center, tapering to spiral arms about 2,000 light years thick. Our galaxy has an estimated 200 to 400 billion stars. How do we know their distance?

Astronomers deduce distance by measuring the brightness of the star, which depends on its color spectrum. This relationship was found from stars close enough to measure by parallax.

A type of variable star — those that change in brightness — also allows distances to be measured even in neighboring galaxies, hundreds of millions of light years away, where individual stars can be detected. They are known as Cepheid Variables, for the first of its class to be discovered — Delta Cephei, a star you can see tonight in the northern sky. These amazing stars vary in brightness in distinct levels directly relating to their period of variability. Knowing the distances of nearby Cepheid Variables from the parallax method, astronomers can tell the distance of a Cepheid much farther away by recording its period of variability and apparent brightness.

Polaris, the North Star, is also a Cepheid Variable. The varying magnitude is not great enough to be noticed to the unaided eye. Delta Cephei, however, can be easily seen to brighten and fade by comparing how it appears to neighboring stars in its constellation, Cepheus. It changes from magnitude 3.6 to a dimmer 4.3 every 5.4 days.

(Brightness of heavenly bodies are ranked by magnitudes; the dimmest star we can see without optical aid is generally magnitude 6. Polaris is approximately 2nd magnitude (1.9). Brilliant orange Arcturus, which shines in the east on May evenings, is magnitude -0.04.)

No matter how far, the stars and planets are as close as your back yard the next clear evening.

Keep looking up!

Peter W. Becker is managing editor at The Wayne Independent in Honesdale, Pa. He has been an amateur astronomer since the age of 12, in 1969.